The present invention relates to a propulsion system for ships, which propulsion system comprises one or several impellers for generating a force that drives the ship forward. The impeller, being rotatable in an impeller house by means of the driving shaft, is provided with blades of the propeller type, which produce the jet stream backwards.
The propulsion of ships, preferably fast moving ships, both military and civilian ones, through water jet arrangement, comprising impellers are generally known. The housing surrounding the rotating impeller provided with blades is fixedly mounted to the rear portion of the hull. The impeller is typically driven by a steel shaft extending towards the stem by suitable arrangements that in turn are driven by one or several engines within the hull. A tube-like water inlet, which slopes somewhat downwards in the moving direction, is provided in front of the impeller housing in order to supply a large amount of water. The driving shaft thus runs through said tubular water inlet. The ship is controlled by means of steering devices downstream the impeller housing (or housings), which may direct the jet stream in different directions. The jet stream may also be directed forwards to give a decelerating effect.
As the driving shaft of the impeller extends through the water inlet, the incoming flow of water to the impeller is disturbed to some extent, which implies that an unevenly distributed load on the blades of the impeller is created. Said uneven load implies that a bending moment is transferred to the impeller inwards towards the attachment point of the impeller. Because of these varying forces influencing the impeller and its attachment point, very high requirements are put on the arrangement of the bearings and scalings. It is known from SE 424 845 (corresponding to U.S. Pat. No. 4,474,561) to solve said problem by arranging the impeller fixedly mounted to the shaft and to arrange a bearing arrangement allowing a certain angle deviation. However, said solution requires a design with a bending rigid driving shaft (in order not to risk too great angle deviations), which design thus is very heavy. It is not unusual that only the weight of the driving shaft in such a design amounts to about 10% of the total weight of the water jet device (including the weight of the pump unit including stator part with guide vanes, thrust and journal bearing arrangement, impeller and impeller housing and the steering and reversing gear). Another known solution is shown in SE 457 165 (corresponding to U.S. Pat. No. 5,045,002) and SE 504 604, wherein a bearing arrangement is used which cannot handle angle deviations and wherein a flexible coupling between the driving shaft and the impeller is used instead, the coupling being intended to handle the angle deviations. Also said last mentioned solution leads to a heavy design, especially since the coupling as such implies an additional weight. Further, it implies a considerable drawback as the coupling is provided at a critical position as to flow, which implies that it is difficult to obtain optimal flow conditions.
The design described in SE 424 845 has satisfactory properties per se, but as mentioned it is heavy because of the rigid, conventional impeller shaft. In certain applications, especially military ones, it is of great importance to reduce the weight and at the same time to obtain optimal flow conditions with devices loaded to a high degree, which implies that conventional water jet design may not be used. Another reason to it not being desirable to use a coupling in connection with such applications is that the coupling implies a power limitation. It is realized that a detail that limits the power transmission is not desirable in such applications, as, especially with such applications, it many times is desirable to be able to transfer a lot of power, often in the interval of 3-30 MW. For long it has been a desire to reduce the weight by replacing the conventional impeller shaft by a lighter shaft and at the same time to eliminate the need of a flexible coupling. Hitherto, that has not been put into practice by anyone.
Indeed it is mentioned in SE 504 604 that the flexible coupling may be eliminated. However, it is not described how this may be achieved. Moreover, there is no indication how the high stresses from a bending rigid shaft might be handled. The design according to SE 504 604 instead shows the use of a flexible coupling and is directed to an embodiment, which makes it possible to dismount the bearing unit backwards. This implies i.a. that the guide vanes, which transmit the force from the impeller to the stator shell, must have a very limited extension. This implies in turn that the possibility of achieving an optimal solution as to weight, flow and strength is limited. Above all, it implies the great drawback that the possibility to transmit very large powers is in principle not practically achievable. Thus, the design does not offer the possibility to good power density (with power density is meant the maximal power output divided with the weight of the water jet unit, comprising the weight of the pump unit including stator part with guide vanes, thrust and journal bearing arrangement, impeller and impeller housing and the steering and reversing gear), i.e. the weight will be comparatively high in relation to the maximal power which may be transmitted. With this design it is probably difficult to achieve a power density above 1.0 kW/kg for a water jet having an inlet diameter above 1 m, which is an undesired and serious limitation. As is evident for the skilled man the power density for the same kind of design does decrease with increased size.
An objective of the invention is to find an optimal solution of the above described complex of problems. Said objective is achieved by a propulsion system for ships comprising an impeller, a stator shell, and an impeller housing for achieving a water jet, a shaft for the propulsion of the impeller, and a bearing arrangement for the shaft in the stator shell, and preferably a sealing of the shaft in the impeller housing, wherein the shaft consists of a light weight shaft, which has considerably lower bending rigidity than a conventional steel shaft, and the driving force is transmitted via at least one non-flexible coupling and via said bearing arrangement which is rigid as to bending and handles the axial load, to the stator shell, such that a high power density is achieved.
Because of the use of a light weight shaft, which becomes comparatively weak as to bending, conditions are created to use a bearing arrangement which is rigid with reference to bending moments and which handles an axial load and at the same time for using non-flexible couplings (e.g. attachment by screws) between the impeller and the end portion of the driving shaft. At the same time, the comparatively weak driving shaft meet the objective to achieve a weight reduction. Further, it makes a cost saving possible with reference to the shaft as the choice of material is optimised in this respect. The shaft may thus be made comparatively slender, and because of the preferred attachment directly against the impeller, optimal conditions are obtained to create as good flow paths as possible, which in turn may imply reduced bending forces influencing the bearing arrangement of the impeller.
According to a preferred embodiment of such a driving system, the driving shaft consists at least mainly of a composite material. Above all, a composite shaft has the great advantage that very low weights may be obtained. A weight reduction of up to 70% as compared to a conventional steel shaft is possible. Further, the advantage is obtained that a composite shaft is exceptionally bendable, which is an advantage with reference to the bearing arrangement. A low bending rigidity is also desirable and a composite shaft may give a reduction of the bending rigidity of about 80% as compared to a conventional, homogenous steel shaft.
According to another aspect, the composite shaft comprises a tubular frame of a first fibrous material, preferably carbon fibre, surrounded by a layer of a second fibrous material, preferably glass fibre, and preferably an outermost erosion protection of an erosion resistant material, preferably polyurethane. As the driving shaft partly lies in the water flow, which may contain some hard and/or abrasive objects, and as a composite shaft, e.g. of carbon fibre, is sensitive to impacts, a preferred embodiment is such a shaft with an impact resistant layer and a protective layer, respectively, which minimises the risk for breakdowns.
According to an additional aspect of the invention, at least some portion of said impeller housing is made of a light weight material, preferably comprising carbon fibre, wherein preferably said portion of the impeller housing is coated with a protective surface, preferably polyurethane. It is the solution according to the invention, which creates the conditions for this additional weight reduction. The reason is that the very bending rigid bearing mounting of the impeller, which in practice is free from play, implies that extremely a good positioning of the impeller blades is obtained with reference to the housing, so that the risk for contact between the ends of the blades and the impeller housing is in principle eliminated. Thus, the solution according to the invention implies that one with larger safety gets the possibility to reduce the weight of the impeller housing, i.e. one may use xe2x80x9cweakerxe2x80x9d and/or thinner material for the impeller housing.
According to further potential aspects:
said bearing arrangement consists of a spherical axial bearing in combination with a conical roller bearing;
the bearings in the impeller housing are lubricated with oil or grease and sealed to the environment by an axially resilient sealing provided in front of the front bearing;
at least one portion of said impeller housing is made of light weight material, preferably comprising carbon fibre;
the inlet diameter D of said impeller housing is between 0.5-2 m and that the power density is at least 0.5+(2xe2x88x92D) kW/kg,
D is between 0.5-1.3 m and that said power density is 0.7+(2xe2x88x92D) kW/kg,
said light weight shaft is made of metal, preferably titanium and/or a hollow steel shaft;
there is no flexible coupling for the transmission of power from the shaft to the impeller,
the inlet diameter D of said impeller housing is above 2 m and the nominal maximum design power is at least 15 MW.
Thanks to the invention, it is possible, as compared to conventional systems, to build a substantially much lighter driving system for a water-jet driven ship and which at the same time provides for a high reliability in operation possible.
The invention will be described more in detail with reference to the accompanying drawing which is a vertical, axial cross section of an impeller and an impeller housing according to a preferred embodiment.